Final answer:
A filler is used to equalize porosity and deposit color in one application to provide a uniform contributing pigment on pre-lightened hair. Hence option B is correct.
Step-by-step explanation:
Fillers are used in the hair coloring process to equalize porosity and deposit color in a single application, helping to create a uniform contributing pigment on pre-lightened hair. Hair fillers typically contain conditioning agents and pigment to address variations in porosity and enhance the overall color result. The use of fillers is particularly beneficial when working with hair that has uneven porosity due to previous color treatments or damage.
Options a) Color wheel, c) Tint brush, and d) Activator are not specifically associated with the equalization of porosity and depositing color in the context described.